Portable lighting is often used in hazardous locations. In the past, incandescent lamps have been widely used in hazardous locations. However, incandescent lamps may break if dropped, thereby exposing the heated filament as well as the electrical power leads to the environment. This may create a potential for an explosion, depending upon the conditions in the environment of which the lamp is used.
Attempts have been made to make incandescent lamps "explosion proof". This has required expensive and elaborate provisions for shielding and enclosing the lamp, and for reinforcing the enclosure for the lamp since the enclosure must be transparent (or at least translucent) and normally is made from special glass. By way of example, one commercial incandescent lamp designed for use in hazardous locations includes a very thick and strong globe of special explosion-proof glass which surrounds the lamp. A metal framework is placed around the globe for coupling to the base of the fixture. These units are expensive, and heavy; and it is time-consuming to replace a burned-out lamp due to the construction of the unit. Further, re-lamping of this type of unit cannot always be accomplished in the "hazardous" location. Sometimes, the lamp must be removed from that location before a new lamp can be installed.
An improved hazardous location lamp using fluorescent electrical lamps is disclosed in the co-pending, co-owned application of Baggio and Granat for AIR PURGED PORTABLE ELECTRICAL LAMP, application Ser. No. 08/431,308. The use of fluorescent lamps over incandescent lamps is known to provide better efficiency. In the improved fluorescent hazardous location lamp, air pressure within the fixture housing is monitored by a low pressure switch and a high pressure switch. These two switches define a design operating range for the pressure inside the fixture. A control circuit includes a timer circuit which commences timing when power is applied to the fixture. Pressurized air (which originally was required to be breathable air, but now must merely be filtered) is applied to the fixture at the same time. The time duration of the timer circuit is set as a function of the known air volume within the fixture. Thus, the start-up cycle for this system requires that the internal pressure be maintained within the design range for a predetermined time to insure that the desired number of volumes of the interior of the fixture will be purged. (For start-up, it is now accepted by the National Fire Protection Association that four times the volume of the interior of the fixture is acceptable for purging to establish safe initial conditions.) If the pressure within the housing is maintained within the design limits, and the timer times out, then power is applied to the lamps.
One problem with this type of fixture, though considered to be an improvement over other available, more costly and heavy incandescent fixtures for hazardous locations, is that safe operation of the system is dependent upon the integrity of the enclosure of the fixture and the integrity and reliability of the pressure switches which are mechanical switches and may stick in one state. For example, if the discharge orifice becomes plugged (which happens in industrial environments), but there is a leak in the end cap of the fixture, through which the breathable air is introduced, then the starting conditions may be established but it is possible that the incoming air is escaping directly into the atmosphere adjacent the air inlet, and the remainder of the fixture (which, in the case of tandem fluorescent lamps may be approximately four feet long), remains unpurged.